Updated Doxygen comments for GPU Matrix Subtract. Also fixed a google code style error in Frobenius Norm.
Updated Doxygen comments for GPU Matrix Subtract. Also fixed a google code style error in Frobenius Norm.

seleanridley/NICE,rockett-m/NICE,rockett-m/NICE,seleanridley/NICE,yiskylee/NICE,yiskylee/NICE,rockett-m/NICE,yiskylee/NICE